Print Preview: Fix some advanced settings bugs

Return the empty string instead of null for unset advanced settings
values. This prevents parse errors in the print ticket for Google Drive.
There is also no way for an advanced setting to be anything other than
a string, since it is set from an <input type="text"> or a <select>.

Also set a min width for the label, to prevent it from disappearing
entirely when the browser window is very narrow.

Bug: 964253, 954119
Change-Id: Ic1e18244a39fac9e7b021afe24a13105f9ed8ff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618646
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661546}
2 files changed